javascript - Purpose of if(0){ code }? - Stack Overflow

admin管理员组

文章数量:1349697

Quick question. I just came across the following in a JS file:

if (0) {
    // code
}

What's the purpose of this if statement? Which cases would the code execute? It currently doesn't run the code in the if clause.

Quick question. I just came across the following in a JS file:

if (0) {
    // code
}

What's the purpose of this if statement? Which cases would the code execute? It currently doesn't run the code in the if clause.

• javascript

Share Improve this question Follow edited Jul 20, 2011 at 19:11 Lightness Races in Orbit 386k7777 gold badges666666 silver badges1.1k1.1k bronze badges asked Jul 20, 2011 at 19:10 Alex HeydAlex Heyd 1,32311 gold badge1010 silver badges1717 bronze badges 5

• 5 Quick way to ment or disable debugging code possibly? – Brad Christie Commented Jul 20, 2011 at 19:11
• Just in case someone does a #define 0 1? – Marc B Commented Jul 20, 2011 at 19:12
• @Marc B: You do know that's impossible in JavaScript don't you? :) – pimvdb Commented Jul 20, 2011 at 19:14
• thanks for all the answers. I've never thought of that way of menting out code. Sorry I can't mark all answers below as the answer =/ – Alex Heyd Commented Jul 20, 2011 at 19:17
• @pimvdb Is there any language where you can #define a numeric token? – Stewart Commented Jul 21, 2011 at 9:55

Add a ment  | 

6 Answers 6

Sorted by: Reset to default 9

It looks like an artifact of the development process. Wrapping code like that gives you a quick, 1-character way to effectively ment out a block of code. It's quicker to toggle than a typical multi-line ment.

Someone has used that to effectively ment out a large chunk of code.

They probably meant to remove it entirely before releasing to the public, but forgot.

Actually, in Javascript, some code runs even in such block. For example, variables being defined in if(0) block will be defined with the value undefined:

if (0)
{
    var f = 1;
}
f; //undefined
g; //ReferenceError

Another good example is the case of declaring functions in blocks. This is undefined behaviour in Ecmascript, so the results may vary across browsers:

if (0)
{
    function f() {}
}
typeof f; //"undefined" in Firefox, "function" in other browsers

IIRC some bulletin board software generates 0 or 1 on the server side; see @eds's post.

It won't run. It could be someone was trying to ment out the code and there were too many /* */ in the middle (or maybe there was some other reason why the original author didn't want to use /* */?). Regardless of the reason, if(0){} means if(false){}. The code between the braces will never run.

that's possible temporary disabled part of code

It's probably just an easy way to toggle enabling/disabling code, but what you may be seeing is JavaScript code that was rendered by a server-side language. So, for example, if you were writing in PHP, and you wanted client-side JavaScript to run if your PHP variable $doThis was non-null, you might write

 if (<? echo $doThis; ?>) {
  // do stuff...
 }

There aren't many situations where this happens, but I know Blogger uses technique where it displays the number of ments below a post.

本文标签： javascriptPurpose of if(0) code Stack Overflow